(1) Field of the Invention
The present invention relates to a method for obtaining polyester resin products having a desired strength, as well as to a mixture used in the method. More particularly, the present invention relates to a method for obtaining polyester resin products having a desired strength by adding a carbodiimide compound to a material polyester resin, as well as to a mixture used in the method.
(2) Description of the Prior Art
In recent years, environmental pollution caused by plastic waste has become a bigger problem, and the necessity for and significance of environmental protection has become more important. Under such circumstances, ways are being sought for recovery and regeneration of used plastic materials for effective reutilization thereof.
In particular, polyethylene terephthalate resin (hereinafter abbreviated to PET resin), which is used as a container for clean drinking water or the like, is currently disposed by reclamation or incineration. Therefore, methods for effective reutilization of used PET resin are being sought from the two aspects of resource saving and environmental protection.
The reutilization methods include a material recycling method which comprises changing the form of a used material into a different form by a physical treatment, for making possible the reutilization.
This material recycling method is actually being applied to the recycling of used PET resin. In the method, however, impurities are easily taken in; therefore, there are problems that the regenerated product is lower in purity, strength and heat resistance than the original product and consequently has limited applications. These problems are considered to be caused by the following reasons.
Ordinarily, when recovered PET resin is processed for reutilization, the recovered PET resin in the form of bottles or the like is first ground into flakes and then processed into products (e.g., pellets). In this operation, the PET resin is deteriorated because it undergoes, in an extruder, hydrolysis (caused by the water which adheres thereto during washing or the like), oxidative decomposition and/or thermal decomposition.
In ordinary resin recycling, recovered resin is dried at 140 to 160.degree. C. for about 5 hours in order to prevent the above-mentioned deterioration. This drying requires a facility investment and a running cost for the facility and, in recycling of used PET resin bottles or the like, incurs a big increase in cost; therefore, the drying is eliminated in some cases, which inevitably invites the deterioration of the PET resin.
There is a further problem. That is, PET resin bottles are produced from PET resins of various intrinsic viscosities (IV) ranging from about 0.6 to 1.2, depending upon the application of the bottles; bottles for soy sauce have an IV of 0.65, bottles for mineral water or tea have an IV of 0.75, bottles for carbonated drinks or the like have an IV of 0.8 to 8.5 and bottles for medical or cosmetic use have an IV of 1.2.
Thus, various PET resin bottles different in properties are collected and ground into flakes at a regeneration plant. Therefore, the flakes have a large fluctuation in properties, and the products, e.g., pellets obtained from the flakes by the use of an extruder are very low in quality.
For the above reason, virgin PET resin must be added to recovered PET resin in order to allow the regenerated product to have a desired strength. Generally, about 30%, based on regenerated product, of virgin PET resin has heretofore been added to recovered PET resin, and the times of reutilization have been about 3. This has made the recycling cost even higher.
Thus, while recovery and reutilization of used polyester resins, particularly used PET resin is required, no satisfactory method therefor is developed yet.